A common technique for manufacturing PCBs employs surface mount technology, where components are secured to conductive pads on the surface of the circuit board. Conductive paths in or on the circuit board interconnect the pads. Components are affixed to these pads by conductive or non-conductive mounting materials, referred to hereinafter as mounting materials. These mounting materials are compounds with preferably both adhesive and electrical conduction properties, and can include materials such as solder paste and conductive epoxy. Solder paste is a viscous paste that includes finely ground solder particles in a flux base. Conductive epoxies are organic compounds which have adhesive properties sufficient to secure electrical components to circuit boards and which are electrically conductive. A variety of conductive epoxies are available, for example, from Epoxy Technology, Inc., Billerica, Mass.
In one known practice, solder paste is printed onto a circuit board through an apertured mask placed in selected registration over the circuit board. A material-applying sweep by a suitable blade, such as a squeegee-like blade, across the mask applies the solder paste to the circuit board in a pattern reflective of the apertures in the mask. Generally, the application of solder paste, or other mounting material, to a circuit board occurs with a single sweep of the blade. The use of multiple sweeps, or multiple blades, is commonly deemed undesirable. After each sweep of the blade, the processed circuit board is removed and replaced with a fresh board. After applying solder paste to the circuit board, leads of various electronic components are placed on the PCB at the sites bearing the solder paste. The solder paste is then heated and allowed to cool, thereby securing the components to the board and connecting them electrically to the conductive runs in the board.
A considerable problem in PCB production relates to the fidelity of applying the mounting material. Fidelity, as used herein, includes resolution, reproducibility and definition. Herein, resolution refers to the minimum distance between two adjacent mask apertures capable of ensuring no inadvertent contact between mounting material sites on the circuit board. Reproducibility refers to the maximum variation in mounting material patterns between any two PCBs produced with the same mask. Definition refers to the precision with which the mounting material pattern reflects the aperture pattern of the mask. Common fidelity problems include, for example, contamination of the mask bottom with mounting material and void spaces in the mounting material deposited on the circuit boards. Void spaces in the deposited mounting material become increasingly pronounced with decreasing size of the mask apertures. Fidelity problems are of significant concern with PCBs designed to accommodate high densities of components, since minute errors in mounting material placement can potentially render these boards useless. Fidelity limitations presently determine the maximum allowable density of PCB components. Increasing the fidelity of mounting material application will allow for the low cost, mass production of PCBs with higher densities of components than is presently possible.
Prior art attempts to increase the fidelity of the mounting material application process include increasing the pressure exerted by the blade upon the mask, using slow blade sweep rates, and using angular blade sweeps across the mask. Each of these techniques suffers from disadvantages. Increased blade pressure decreases mask life, increases blade wear, and scavenges mounting material from mask apertures. Scavenging of mounting material occurs when the blade, during the sweep, dips into an aperture and scoops out material from that aperture. Other PCB production problems amplified by increased blade pressures are “screen stretch” and contamination of the mask bottom with mounting material. “Screen stretch” is caused by friction between the blade and the mask, which stretches the mask in the direction of the sweep. Contamination of the bottom of the mask with mounting material results in mounting material being applied outside the desired mask pattern and requires cleaning of the mask.
Slow blade movement across the mask may somewhat increase the resolution and definition of mounting material deposition, however, it increases the time to process each circuit board and hence reduces manufacturing rates and increases costs.
The use of an angular sweep to spread mounting material across a mask has also been employed in an effort to increase the fidelity of mounting material application. One application of this method involves sweeping a blade, which is set at an angle relative to the sweep direction, down the length of the mask so that the blade passes over the mask apertures, which generally are placed on X and Y axes, at oblique angles. This method of applying mounting material, however, is impractical for the mass production of PCBs due to spacial constraints. To prevent shifting of the mask in a typical current practice, a four-sided frame of minimal size holds the mask tightly in place by securing it on all four sides. As such, when a blade makes an angular sweep, limitations on available space result in solder paste not being applied to two corners of the mask surface. Further, this conventional technique also leads to inconsistent results especially when using fine pitch pads that are disposed perpendicular to each other. The length of the sweep stroke is also lengthened considerable.
In order to address these and other issues, the assignee hereof invented a method whereby a vibrating force was applied to the blade during use. An example of this method is described and illustrated in U.S. Pat. No. 5,522,929 of Erdmann, the contents of which are herein incorporated by reference. As described therein, the blade oscillates along a single axis that is generally transverse to a sweep direction. The blade thus travels in a zig-zag motion along this axis, and is an effective method for evenly filling and leveling stencil or mask apertures. A drawback of this method is that the frequency of the blade oscillation is fixed during use.